Printed circuit boards may take on multiple functional compositions, essentially concerning the number of metal sheets with an image of the circuit they form. So, for example, and among other forms, a board composition is initially formed from a metal sheet and a layer of insulating material impregnated with polymerizable resin on which, once the resins are polymerized and the board is cooled until it is hard, an engraved image of the circuit is formed on the metal sheet; another board composition is formed initially by two metal sheets between which there are one or more layers of insulating material; and a third composition for a printed circuit board is initially formed by two metal sheets between which there are two layers of insulating material separated by a printed circuit board provided on both faces with the already engraved image of the circuit.
In general, the process of polymerizing the resins in the layers of insulating material is carried out by subjecting the board whose composition has been disclosed to pressure at the same time as the temperature is raised until it reaches polymerization temperature. To do this, presses are used which are essentially differentiated by the method of heating the boards. One type of press includes means of heating arranged on the press plates of the press, so that the board is heated during pressing by thermal conduction. Another type of press includes means of heating based on passing an electric current supplied by a power source—generally direct current—through the board's sheets; in this case, the metal sheets are arranged continuously in order to make it possible for the electric current to pass through.
There are drawbacks with the types of press disclosed above. In presses wherein the means of heating are arranged on the press plates, the rate of temperature increase and the maximum temperatures that can be reached are very limited, which leads to low productivity. And, in presses where the means of heating are based on passing an electric current through the board's metal sheets, problems of electrical insulation between these and intermediate press plates arise.